In many situations, physical variables are monitored and an alarm is sounded when one or more of those variables go outside of preset limits or when a specified event occurs. Examples of such systems are fire alarms and burgular alarms, commonly known jointly as fire and security systems. A critical requirement of such systems is that the alarm circuit be maintained functional at all times. In order to insure that the alarm circuit is always functional, many such alarm circuits include operation in a supervisory mode which checks for open circuits which might be caused by a break in the wiring or a loose connection. A further precaution that is taken in many systems is the use of what is known as a "class A" circuit. In a class A circuit, an open circuit condition will be detectable by the supervisory mode, but will not prevent the alarm from sounding should an alarm condition occur before an open circuit has been repaired.
The use of supervision and class A operation greatly enhances the reliability of such alarm circuits. A problem which still exists, however, lies in the fact that if a circuit is miswired, the supervisory mode may indicate that the circuit is in operational condition, but when an alarm condition occurs, the alarms will fail to be activated.